A propagating wireless signal can be affected by different materials in different ways prior to arriving at a destination such as a receiving device. For example, different materials may cause the wireless signal to be reflected, scattered, diffracted, and/or absorbed, among other possibilities. Wireless signal absorption is generally characterized by a reduction in the signal power as a portion thereof is converted into heat within the material causing the absorption. The detection of such wireless signal absorption is useful in several applications. In some instances, a material may be characterized based on its frequency-dependent absorption properties. For example, a transmitter positioned near the material may vary the frequency of a transmitted signal over a range of interrogation frequencies, and a receiver positioned on the other side of the material may detect the frequency response.
Using wireless signal absorption to estimate the number of people surrounding or near a receiving device is problematic for a number of reasons. First, methods may generally require knowledge of the transmitting signal power and/or the position of the transmitter, thereby requiring a system-controlled transmitter at a fixed position. Second, counting a number of people based on a number of wireless signals can be highly inaccurate, where the proportion of the crowd with devices that emit signals can vary drastically, depending on the demographics of the crowd. This may also require some identification of the signal sources (to ensure they are uniquely identified), which may require decoding signal data (e.g., to determine an identifier). Accordingly, new systems, methods, and other techniques are needed.